Chemical aids for paper play an important role in the sustainable development of the paper-making industry, and therefore attract extensive attention. Chemical aids in paper-making can be classified into processing aids and functional aids. One of the functional aids is the strength aids, including dry strength aids, wet strength aids and temporary wet strength aids, etc.
A polymer of glyoxylated polyacrylamides, GPAMs, is one of frequently used temporary wet strength aids, as well as one of frequently used dry strength aids (see, e.g., U.S. Pat. No. 3,556,932A, U.S. Pat. No. 4,605,702A, U.S. Pat. No. 5,674,362A, U.S. Pat. No. 6,245,874B1, WO0011046A1, U.S. Pat. No. 7,641,766B2 and U.S. Pat. No. 7,901,543B2).
GPAMs are provided mainly in the form of a polymer solution. Generally speaking, if having the same solid content in the polymer solution, GPAMs with a higher molecular weight can provide a better temporary wet strengthening effect. However, the higher the molecular weight, the easier the GPAMs become gelled, so that the shelf life of the polymer solution is shortened, limiting the practical application of the polymer solution in the paper-making process. In order to secure that the polymer solution has a suitable shelf-life, usually it is necessary to (1) reduce the solid content of GPAMs in the polymer solution, or (2) reduce the molecular weight of GPAMs. As for (1), it requires that the solid content of the GPAMs in the polymer solution remains constant, normally between 8 and 20 wt %, for the sake of production and transportation. As for (2), more GPAMs with a lower molecular weight are required to be added into the pulp, compared to the GPAM with the higher molecular weight, in order to realize a comparable temporary wet strengthening effect, which is obviously not economical. In particular, the GPAMs with a weight average molecular weight of from 100,000 to 300,000 Dalton, despite its longer shelf life, has no practical application in the industry due to its unsatisfactory strengthening effect.
Therefore, how to endow the GPAMs with a better temporary wet strengthening effect without increasing the dosage of the GPAMs is a problem to be resolved.
On the other hand, amphoretic polyacrylamides are one of the frequently used dry strength aids (see, e.g., JP1049839B), while they almost have no effect of increasing the temporary wet strength.
WO9806898A1 discloses a paper-making process, wherein cationic polymer selected from the group consisting of cationic starch and cationic wet strength resin, and amphoteric polyacrylamide-type polymer is added to the aqueous pulp slurry in order to increase the dry strength of paper, and wherein GPAM can be used as the cationic wet strength resin. Moreover, U.S. Pat. No. 6,294,645B1 discloses a dry-strength system for paper comprising PAE, amphoteric PAM and wet strength resin, wherein GPAM can be used as the cationic wet strength resin. However, the above prior art literatures neither disclose nor suggest the combination use of GPAM and the amphoteric PAM has an effect of increasing the temporary wet strength, not to mention disclose or suggest the influence of the molecular weights of GPAM and the amphoteric PAM as well as the selection of their ratios on the effect of increasing the temporary wet strength.